The Basics of Carbohydrate Digestion
Carbohydrates are a primary energy source, but before your body can use them, they must be broken down into their simplest forms, or monosaccharides. This digestion process begins in the mouth with salivary amylase, pauses in the acidic stomach, and is completed in the small intestine by pancreatic amylase and enzymes on the intestinal wall. The final products—glucose, fructose, and galactose—are then absorbed into the bloodstream through specialized transport proteins.
The Limiting Step in Absorption
For many years, it was believed that the body’s maximum rate of carbohydrate absorption was approximately 60 grams per hour. This limit is due to the saturation of the sodium-glucose cotransporter (SGLT1) protein, which is responsible for transporting glucose and galactose from the small intestine into the bloodstream. When an athlete consumes too much glucose-only fuel beyond this limit, the intestinal transport system can become overwhelmed, leading to gastrointestinal issues such as bloating, cramping, and diarrhea.
Overcoming the 60g/hr Glucose Barrier
The discovery of different intestinal transporters for different types of sugar revolutionized fueling strategies for endurance athletes. By consuming a combination of carbohydrates that use separate absorption pathways, athletes can increase their total hourly intake.
The Role of Multiple Transportable Carbohydrates
Research demonstrated that by combining glucose (which uses the SGLT1 transporter) with fructose (which uses the GLUT5 transporter), the overall absorption rate could be significantly increased. Fructose, which is absorbed more slowly, uses a different intestinal pathway, allowing it to be absorbed independently of the glucose transport system. A common ratio used is 2:1 (glucose to fructose), which can enable absorption rates of up to 90 grams per hour. More recent studies have even explored higher ratios, such as 1:0.8 or 1:1, showing potential for even higher absorption rates up to 120 grams per hour for elite athletes during extreme efforts.
How to Fuel with Combinations
To leverage multiple transportable carbs, athletes can use specially formulated sports drinks, gels, or chews that provide an optimal glucose-to-fructose ratio. For example, a sports drink containing both maltodextrin (a form of glucose) and fructose allows for higher total carbohydrate intake than a drink with only one sugar source. The source of the carbohydrate (liquid, semi-solid, or solid) doesn't significantly impact absorption rate, as long as the products are high-quality, low in fiber, and easily digestible.
Comparison of Carbohydrate Absorption
To illustrate the difference in absorption limits and the role of combining carbohydrate types, the following table breaks down the capabilities:
| Carbohydrate Type | Primary Transporter | Hourly Absorption Limit | Example Foods/Sources | Target Athlete Type |
|---|---|---|---|---|
| Glucose Only | SGLT1 | ~60g/hr | Maltodextrin, Dextrose, White Bread | Shorter Efforts (1-2 hours) |
| Fructose Only | GLUT5 | ~30g/hr (variable) | Fruit, Fruit Juices | General Consumption |
| Glucose + Fructose (2:1) | SGLT1 & GLUT5 | ~90g/hr | Most Sports Gels, Drinks | Endurance Sports (>2 hours) |
| Glucose + Fructose (1:0.8 to 1:1) | SGLT1 & GLUT5 | >90g/hr (100-120g) | Elite Sports Fueling | Elite Ultra-Endurance |
| Complex Carbs (Starch) | Amylase breakdown first, then SGLT1/GLUT5 | Varies (slower release) | Legumes, Whole Grains, Sweet Potatoes | General Nutrition |
Pushing the Limits: Gut Training for Athletes
Just like training your muscles, your digestive system is also adaptable. For athletes aiming for the highest levels of carbohydrate intake—90g to 120g per hour—gradual adaptation through “gut training” is essential to avoid gastrointestinal discomfort.
What is Gut Training?
Gut training involves deliberately consuming high amounts of carbohydrates during training sessions to increase the efficiency and capacity of the gut's absorption transporters. This can help improve gastric emptying and reduce the risk of GI issues on race day. This adaptation must be built up slowly over a longer period, allowing the body to adjust to the increased intake.
Practical Tips for Adaptation
- Start Small: Begin by practicing with lower intake rates (e.g., 60g/hr) during your long training sessions before increasing gradually.
- Stay Consistent: Regular exposure to high carbohydrate loads is key for adaptation. Make fueling a routine in your training plan.
- Mix Your Sources: Use a variety of carbohydrate sources (gels, chews, drinks) to find what works best for your body and to ensure you're utilizing both glucose and fructose transporters.
- Listen to Your Body: Pay close attention to how you feel. Any signs of GI distress mean you should scale back and adjust your strategy.
Potential Pitfalls: Gastrointestinal Distress
Consuming more carbohydrates than your body can absorb is a common cause of GI problems during exercise. Symptoms can include bloating, nausea, cramping, and even diarrhea. Factors contributing to this include:
- Ingesting too many single-source carbs: Exceeding the 60g/hr glucose limit can overwhelm the SGLT1 transporters.
- High fructose intake without glucose: Consuming too much fructose alone can lead to malabsorption and discomfort.
- Dehydration: Insufficient fluid intake can slow gastric emptying and exacerbate GI issues.
- High intensity: During very intense exercise, blood flow is diverted from the gut to the muscles, which can inhibit digestion and absorption.
- Lack of training: An untrained gut is more susceptible to distress when faced with high fueling rates.
Conclusion: Fueling Smarter, Not Just Harder
Ultimately, the question of how many carbs can your body digest in an hour depends heavily on an individual's training status, the duration and intensity of exercise, and the specific types of carbohydrates consumed. While a standard intake of 60 grams per hour is a safe baseline for many, endurance athletes can increase this to 90 grams or even up to 120 grams per hour by strategically combining glucose and fructose sources. By understanding the physiological limits and adapting through gut training, athletes can optimize their fueling strategy to maximize performance and avoid digestive setbacks. Read more on carbohydrate intake for endurance athletes.
